Product containing alumina trihydrate and a source of B2 O3 and method

ABSTRACT

The ASTM E84 flamespread of fibrous products such as hardboard is reduced by incorporation of a combination of alumina trihydrate and a source of B 2  O 3 , such as boric acid. The B 2  O 3  source should be capable of providing B 2  O 3  at a temperature below 400° C.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending application Ser. No.588,687, filed June 20, 1975, now abandoned.

FIELD OF THE INVENTION

This invention relates to a fire retardant, reconstituted wood-basedproduct containing alumina trihydrate and a source of B₂ O₃ capable ofproviding B₂ O₃ at a temperature lower than about 400° C. The B₂ O₃ canbe provided by a number of compounds such as boric acid, ammoniumpentaborate, ammonium biborate, or other ammonium borate compounds. Thecombination of alumina trihydrate and a source of B₂ O₃ synergisticallyprevents the spread of flame along man-made, wood-based product.

BACKGROUND OF THE INVENTION

Finding safe, reliable and inexpensive means for making noncombustibleproducts is one of the most important problems facing many industriestoday. Regulations relating to the flammability of building products,textiles, appliances and the like are becoming much more stringent, andwill continue to be more stringent as the public is made more aware ofthe hazards presented by combustible materials. The hardboard industryis no exception to this trend, and the use of hardboard paneling iscurrently being restricted because of its inherent flammability.

The standard test for the flammability of building products is the ASTME84 (Steiner tunnel) test. By this test, materials are rated for: (1)flamespread index (FSI) (2) fuel contribution, and (3) smoke generation.All materials are rated relative to asbestos cement board (0 on allindices) and red oak (100 on all indices). Hardboard typically receivesa flamespread index of 175-200 (Class III), a fuel contributed value of150, and a smoke index of 400-600. In contrast, a low-hazard materialsuch as gypsum board typically receives a flamespread of 10-15 (ClassI), a fuel contributed value of 15-30 and a smoke index of 0. While allthree indices are important, flamespread and smoke are the majorconcern. Class I products have indices of 0-25, Class II products rangefrom 26-75 and Class III materials have indices between 76 and 200.

In testing various materials in the laboratory, a 2 feet standardizedlab tunnel, commonly known as a Monsanto tunnel (J. Paint Technology,Vol. 39, No. 511, page 494, 1967, and Vol. 46, No. 591, pages 62-69,1974) is used. This tunnel consists basically of a 24 × 4 inch angleiron frame inclined at 28° from the horizontal into which the boardsample (231/2 × 33/4 × 0.2-0.3 inch) is placed. A gas burner (FisherScientific No. 3-902) is mounted so that the burner flame impinges thelower end of the board. The maximum distance that the burning flame tipmoves up the inclined board under controlled gas flow in four minutes isreported. A reliable correlation has been found between the Monsanto 2feet tunnel and the 25 feet Steiner tunnel. If the longest flame lengthin the Monsanto tunnel during the 4 minute test is 12-13 inch the panelwould attain a Class I rating in the E84 test. Flamelengths up to 17-18inch correspond to Class II materials. No smoke or fuel measurementshave been made on the Monsanto tunnel. The Steiner tunnel uses both timeand distance to determine flamespread indices and due to the method ofcalculation many treatments will reduce the flamespread index ofhardboard to the 90-110 range. It is much more difficult, however, toreduce the flamespread index from 100 to 75 since the flammability mustalmost be reduced by an additional one-half to move these last 25 pointsinto the Class II range. The synergistic mixture of alumina trihydrateand borate readily reduces the flamespread index of hardboard to theClass I or Class II level.

Several approaches have been taken to reduce the flame-spread ofhardboard in previous years when flammability was not quite as crucial aconcern. Physical characteristics such as caliper, specific gravity, andembossing were examined for their effects on flamespread and it wasfound that denser, thicker panels received the lowest flamespreads,while embossing had little effect. Asbestos and metal overlays weretried as a means of reducing flamespread, but with little success. Itbecame evident that flamespread reduction was more than just a surfacephenomenon, involving instead the characteristics of the entire board.With this in mind, a variety of commonly accepted chemical treatmentswere attempted, but none could sufficiently reduce the flamespread to anacceptable level without adversely affecting moisture sensitivity andother board properties. Severe production problems were also common tothese treatments.

More recently attention has been diverted from chemical treatments tothe concept of fuel dilution. As the term suggests this involvessubstituting noncombustible (generally inorganic) materials for woodfiber until the desired flamespread reduction is achieved. Little workhad been done previously with this concept since it is normallyconsidered an inefficient mechanism, requiring too much expensivesubstitute material. Dilution levels of 65-80% are not unusual to attainselect levels of fire retardance. In arriving at the present invention,an extensive evaluation of various diluents was undertaken in thefabrication of hardboard. Materials such as fly ash, cement, vermiculiteore, slag, and mineral wool were examined. Mineral wool was selected asthe most likely candidate for investigation since it was fibrous innature and low in cost and an extensive evaluation of fuel dilution inhardboard followed. As suspected, very high dilution levels on the orderof 75% (75 parts mineral wool -- 25 parts wood fiber) were required toachieve a Class II flamespread rating. At such low wood fiber levels thephysical properties of the board were very poor, so an extensive amountof work was done to bring these properties up to an acceptable level.Eventually, acceptable interior and exterior Class II hardboardformulations were developed in the laboratory but attempts to producesuch formulations in the plant met with little success because themineral wool proved too fragile for existing equipment and quickly brokedown into fines. Low levels were added successfully, but at high levelsthe wetlap became muddy and unhandleable. Dry process formulations weredeveloped in the laboratory but were not run at the manufacturingplants.

Work with the mineral wool indicated that if substitution levels couldbe reduced to 50% or less the chances for success with fuel dilution onexisting plant equipment would be much better. Studies showed that therewas little difference between inert fuel diluents except as flamespreadwas affected by inadequate retention and distribution. What was neededtherefore was a low-cost fuel diluent that would provide an activecontribution to flamespread reduction of the remaining combustiblefibers in addition to its passive roll as a diluent.

One such diluent meeting these requirements was alumina trihydrate, Al₂O₃.3H₂ O. This white powdery material is approximately 35% by weightwater and exhibits a considerable endotherm in the same temperaturerange at which hardboard begins to undergo severe pyrolysis. Little orno water of hydration is lost during hot pressing. Thus, this materialcould be used as an active fuel diluent and on the basis of its thermaldata should be more effective than mineral wool. This was subsequentlyshown to be true as 40-45% alumina trihydrate was as effective as 65-75%mineral wool. This research work was then moved to the plant trialstage.

During the initial alumina trihydrate plant trials boards with 40-45%diluent were targeted but these levels were not reached because ofwetlap handling problems. No Class II hardboard was made since 32-34%alumina trihydrate was the maximum level attained. Products with thislevel attained low Class III flamespread ratings in the 25 feet tunnel.As the program progressed into its later stages and operating personnelbecame more familiar with running this diluent, the 40-45% levels werereached; but it was believed necessary in the initial stages to limitthe alumina level to 32%, so a means of further increasing itseffectiveness was sought. This led to examination of mixtures ofchemicals and fuel diluents in an effort to combine the expertise ineach of these areas into one cause.

DESCRIPTION OF PRIOR ART

It is well known to employ boron compounds such as boric acid, B₂ O₃,and ammonium borates for the purpose of flame-proofing productscontaining one or more flammable fibrous components. It is further wellknown to use alumina trihydrate (Al₂ O₃.3H₂ O or Al(OH)₃) as aflame-proofing component in the production of fibrous products. However,before the present invention, no one has used the combination of a B₂ O₃source, selected from the group consisting of boric acid, B₂ O₃, and anammonium borate, together with alumina trihydrate for fire proofing acombustible fibrous product.

The following patents disclose the use of boron compounds forfire-proofing a product containing a flammable fibrous component: Trent-- U.S. Pat. No. 87,005; Eichengrun -- U.S. Pat. No. 1,612,104; Vivas --U.S. Pat. No. 1,612,676; Edwards -- U.S. Pat. No. 1,778,147; Vivas --U.S. Pat. No. 1,839,136; Desper -- U.S. Pat. No. 1,879,128; Wiener et al-- U.S. Pat. No. 1,937,679; Quinn -- U.S. Pat. No. 2,030,653; Becher --U.S. Pat. No. 2,108,761; Hopkinson -- U.S. Pat. No. 2,250,483; Cook etal -- U.S. Pat. No. 2,381,487; Jones et al -- U.S. Pat. No. 2,452,055;Jones -- U.S. Pat. No. 2,523,626; Niesen -- U.S. Pat. No. 2,526,083;Farber -- U.S. Pat. No. 2,573,253; Lauring -- U.S. Pat. No. 2,594,937;Van De Zande -- U.S. Pat. No. 2,769,729; Lauring -- U.S. Pat. No.2,849,316; Dunn et al -- U.S. Pat. No. 2,875,044; Hunter et al -- U.S.Pat. No. 3,028,411; Hunter et al -- U.S. Pat. No. 3,131,071; Videen --U.S. Pat. No. 3,202,570; Behr et al -- U.S. Pat. No. 3,220,858; Orth etal -- U.S. Pat. No. 3,245,870; Pataski et al -- U.S. Pat. No. 3,321,421;Cooper et al -- U.S. Pat. No. 3,367,863; Draganov -- U.S. Pat. No.3,378,381; Chase -- U.S. Pat. No. 3,438,847; Humphrey -- U.S. Pat. No.3,524,761; Dunay et al -- U.S. Pat. No. 3,553,132; Ashton -- U.S. Pat.No. 3,560,253; Winters et al -- U.S. Pat. No. 3,560,351; Bell -- U.S.Pat. No. 3,619,352; Riem et al -- U.S. Pat. No. 3,629,005; Woods -- U.S.Pat. No. 3,816,307; Cavazos et al -- U.S. Pat. No. 3,819,517; Arthur Jret al -- U.S. Pat. No. 3,821,196.

The following patents disclose the use of alumina hydrate or aluminatrihydrate for making a fire-retardant, flammable fibrous product:Becher -- U.S. Pat. No. 2,108,761; Becher -- U.S. Pat. No. 2,611,694;Sanderford et al -- U.S. Pat. No. 3,699,041; Burton -- U.S. Pat. No.3,741,929; and Nicodemus et al -- U.S. Pat. No. 3,772,455. Of the abovepatents, the Cook et al -- U.S. Pat. No. 2,381,487; Orth et al -- U.S.Pat. No. 3,245,870 and Cooper et al -- U.S. Pat. No. 3,367,863 patentsteach the use of boron compounds with alumina trihydrate.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a man-made cellulosicproduct having reduced flamespread characteristics.

Another object of the present invention is to provide a man-made,wood-based product with a synergistic combination of alumina trihydrateand a B₂ O₃ source selected from the group consisting of boric acid, B₂O₃, and an ammonium borate.

Another object of the present invention is to provide a man-made,wood-based product having new and reduced flamespread characteristicsresulting from the combination of alumina trihydrate in an amount ofabout 10-60% by weight of the product, and a boron compound yielding B₂O₃ at a temperature below about 400° C.

Another object of the present invention is to provide a new and improvedman-made, wood-based, product having vastly improved moisture resistanceas compared to the use of common fire retardants well known in thetrade.

Another object of the present invention is to provide an inexpensiveman-made, wood-based product by materially reducing the amount ofalumina trihydrate necessary for fabricating an ASTM E84 -- Class IIflamespread rated product (having a flamespread index (FSI) between 26and 75) by including a minor amount of a boron compound capable ofyielding B₂ O₃ at a temperature less than about 400° C.

Another object of the present invention is to provide a Class I FSIrated man-made, wood-based product (FSI less than 25) whileincorporating less than 45% by weight alumina trihydrate.

In brief, the above and other objects and advantages of the presentinvention are achieved by incorporating both alumina trihydrate and asource of B₂ O₃ in a man-made, combustible, wood-based product such ashardboard. Quite surprisingly, it was found that the combination of asource of B₂ O₃ and alumina trihydrate synergistically lowers theflamespread index on man-made, wood-based products. The results offlamespread tests on such man-made cellulosic boards containing aluminatrihydrate and a boron compound that yields B₂ O₃ at a temperature below400° C. are fed better than the additive results achieved by eachcomponent alone. This is especially surprising when the non-linearnature of the standard ASTM E84 test is taken into consideration. Theresulting board also has excellent moisture-resistance properties and inmany ASTM E84 tests no measurable smoke is detected during the test.

When the new and improved man-made fibrous product of the presentinvention is made by the wet process, the alumina trihydrate is added toa slurry of water and wood fiber. The B₂ O₃ source is added to thesurface of the wetlap during primary manufacture of the product or isimpregnated into the completed product in a secondary operation. Commontechnology well known to those in the trade is used. In this manner, theB₂ O₃ source is drawn into the wetlap from the surface and is evenlydistributed throughout the board during drying and consolidation.Generally, consolidation is achieved by hot-press conditions well knownin the art for the particular type of product made.

When this new product is made by the dry process, the alumina trihydrateand B₂ O₃ source can be premixed in powder form and added at anyconvenient point to the wood fiber. The alumina trihydrate and B₂ O₃source may also be added separately in powder form during the primarymanufacture of the product or else the alumina trihydrate may be addedto the wood fiber and the B₂ O₃ source added via a post-press solutionimpregnation. Here again the art of vacuum-pressure impregnation is wellknown to those in the trade. More of the synergistic mixture of aluminatrihydrate and B₂ O₃ source may be added to the center of the productthan the faces but the exact distribution is not critical with respectto the uniqueness of the synergism noted. Similar synergistic responseshave been noted in medium-density fiberboards and particle or flakeboardcommon to the trade. In any process the method or order of addition ofthis synergistic mixture is not critical.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The new and improved wood-based, man-made fibrous product of the presentinvention generally comprises a flammable fibrous component, such ascellulosic fibers, alumina trihydrate in an amount of about 10-60% byweight of the finished product, and a B₂ O₃ source in an amount of about1/2-7% by weight of the finished product. The flammable fibrouscomponent generally comprises about 20-90% by weight of the finishedproduct. To achieve the full advantage of the present invention, the B₂O₃ source should be capable of providing B₂ O₃ at a temperature lessthan about 400° C. and particularly at or below the pyrolysistemperature of the product.

When the source of B₂ O₃ is boric acid, it is preferred to include abasic buffer, such as borax, in an amount approximately equal to theamount of the B₂ O₃ source. The buffer will prevent undue hydrolysis ofthe board in localized areas of boric acid accumulation and aids insolubilizing the boric acid. During hot-pressing, the localizedaccumulations of boric acid could otherwise result in charring of theboard. The buffer is not essential, especially when the B₂ O₃ source isevenly distributed throughout the product. The buffer need not bepresent to achieve the synergistic flamespread reductions provided bythe combination of alumina trihydrate and a B₂ O₃ source.

In the research program leading to the present invention, many fueldiluents such as mineral wool, fly ash, vermiculite, clays, Portlandcement, perlite, diatomaceous earth, and the like were examined butalumina trihydrate was superior with its inherent water of hydration andendothermic properties compatible with wood. Many aluminatrihydrate/wood fiber Class II (FSI 75 or less) boards were made withoutproblem in the laboratory but early attempts to run similar formulationson production lines were thwarted due to unanticipated problems. Afterconsiderable experience, operating personnel were able to run suchformulations successfully but these early failures provided a strongimpetus to minimize the weight percent needed of alumina trihydrate.Combining the alumina with other well known fire-retardant chemicals wasthe first facet of that research program and led to the surprisingdiscovery of the alumina trihydrate -- B₂ O₃ synergism in cellulosicbased materials. All experimentation has shown that the lower thealumina additive level the fewer the production problems and the moresatisfactory the resulting properties.

In attempting to supplement the flame resistance characteristics ofalumina trihydrate with other well known flame retardent compounds, itwas found that many of the well known compounds are actually detrimentalto the flame-resistance characteristics of the board containing aluminatrihydrate. As set forth in Table I some of the detrimental flameretardant compounds are diammonium phosphate and monoammonium phosphate.Others such as zinc chloride and zinc borate give no improvement or, atbest, the results are merely additive. Boric acid and the ammoniumborates, however, produced remarkable and synergistic flamespreadreduction when combined with alumina trihydrate. The useful boric acidsare at least partially hydrated since even anhydrous boric acid isquickly partially hydrated by picking up moisture from the air. Thepartially dehydrated anhydrous boric acid was rehydrated by allowing aboard containing anhydrous boric acid to reach its equilibrium moisturecontent of about 7% by standing for about 8 hours. The partiallydehydrated boric acid was partially dehydrated by heating a boardcontaining boric acid in an oven at 105° C. for a period of about 8hours. The boards tested in obtaining the results of Table I were madeby the dry process to achieve more uniform alumina trihydratedistribution but the same results are noted on wet process board.

                  Table I                                                         ______________________________________                                        Flamelength Reductions in Blends of Alumina Trihydrate                        and Fiber Due to the Addition of Various Fire Retardant                       Chemicals                                                                                 Monsanto   Flamelength                                                                              Flamelength                                 Composition* %                                                                            Tunnel     of         Net                                         by weight of total                                                                        Flamelength                                                                              Control    Change                                      ______________________________________                                        Monoammonium                                                                  phosphate (5%)                                                                                17"    15"        +2"                                         Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   Diammonium                                                                    phosphate (5%)                                                                                161/2" 141/2"     +2"                                         Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   Zinc chloride (5%)                                                                            15"    141/2"     +1/2"                                       Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   Zinc borate (5%)                                                                              14"    141/2"     -1/2"                                       Alumina                                                                       trihydrate (47.5%)                                                            Fiber (47.5%)                                                                 Sodium borate (5%)                                                                            131/2" 141/2"     -1"                                         Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   Partially rehydrated                                                          anhydrous boric                                                               acid (5%)       14"    161/2"     -21/2"                                      Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   Partially dehydrated                                                          boric acid (5%)                                                                               131/2" 17"        -31/2"                                      Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   Boric acid (5%)                                                                               101/2" 141/2"     -4"                                         Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   Ammonium                                                                      pentaborate (5%)                                                                              10"    15"        -5"                                         Alumina                                                                       trihydrate (45%)                                                              Fiber (50%)                                                                   ______________________________________                                         *The fiber was the dry pressure refined type described in Table X        

Since these results were quite surprising, boric acid was investigatedfurther. Table II shows the effect of the addition of 2.5% boric acidbased on the total weight of alumina trihydrate plus fiber and variousratios of alumina trihydrate to dry-process fiber board made from thepressure refined fiber described in Table X. As can be readily seen,this small boric acid add substantially reduced the amount of aluminatrihydrate required for a Class I or II product.

                  Table II                                                        ______________________________________                                        Monsanto Tunnel Tests of Dry Process Hardboard                                Containing Various Levels of Alumina Trihydrate                               and Boric Acid                                                                             Monsanto Tunnel                                                               Flamespread Length**                                             Composition*   Without Boric                                                                              2.5% Boric Acid                                   (% by weight)  Acid Add     Added                                             ______________________________________                                        Alumina trihydrate (60%)                                                                       121/2"       --                                              Fiber (40%)                                                                   Alumina trihydrate (55%)                                                                       141/2"       --                                              Fiber (45%)                                                                   Alumina trihydrate (50%)                                                                       15"          11"                                             Fiber (50%)                                                                   Alumina trihydrate (40%)                                                                       171/2"       13"                                             Fiber (60%)                                                                   Alumina trihydrate (30%)                                                                       191/2"       16"                                             Fiber (70%)                                                                   Alumina trihydrate (20%)                                                                       23"          181/2"                                          Fiber (80%)                                                                   ______________________________________                                         *The fiber was the dry process pressure refined type described in Table X     **The boric acid was a granular form added to the blend of alumina            trihydrate and fiber.                                                    

Table III shows the effects of various add levels of boric acid to 50/50and 30/70 (alumina trihydrate, dry-process, hardboard fiber) blends.

                  Table III                                                       ______________________________________                                        Monsanto Tunnel Tests of Dry Process Hardboard                                Containing Various Levels of Boric Acid at Constant                           Alumina Trihydrate Level                                                      Composition*             Monsanto Tunnel                                      (% by weight)            Flamespread Length                                   ______________________________________                                        Alumina trihydrate (50%) 14"                                                  Fiber (50%)                                                                   Alumina trihydrate (50%)                                                                     plus 1/2%**                                                                             131/2"                                               Fiber (50%)    boric acid                                                     Alumina trihydrate (50%)                                                                     plus 1%   13"                                                  Fiber (50%)    boric acid                                                     Alumina trihydrate (50%)                                                                     plus 11/2%                                                                              111/2"                                               Fiber (50%)    boric acid                                                     Alumina trihydrate (50%)                                                                     plus 2%   111/2"                                               Fiber (50%)    boric acid                                                     Alumina trihydrate (50%)                                                                     plus 21/2%                                                                              111/2"                                               Fiber (50%)    boric acid                                                     Alumina trihydrate (50%)                                                                     plus 5%   101/2"                                               Fiber (50%)    boric acid                                                     Alumina trihydrate (30%) the flame traveled                                   Fiber (70%)              the length of the tunnel                                                      (24") in 3 minutes                                                            30 seconds                                           Alumina trihydrate (30%)                                                                     plus 11/2%                                                                              21"                                                  Fiber (70%)    boric acid                                                     Alumina trihydrate (30%)                                                                     plus 21/2%                                                                              201/2"                                               Fiber (70%)    boric acid                                                     ALumina trihydrate (30%)                                                                     plus 5%   18"                                                  Fiber (70%)    boric acid                                                     Alumina trihydrate (30%)                                                                     plus 10%  16"                                                  Fiber (70%)    boric acid                                                     ______________________________________                                         *The fiber was the dry process pressure refined type described in Table X     The boric acid was a granular form added to the alumina trihydrate and        fiber slurry.                                                                 **Additions of boric acid based on % by total weight of alumina trihydrat     and fiber.                                                               

A mixture of ammonium pentaborate/boric acid may also be suitable foraddition to the alumina trihydrate/wood fiber mixture.

Tests made on boards containing various percentages of aluminatrihydrate and ammonium pentaborate gave the flame-spread reduction dataset forth in Table IV.

                  Table IV                                                        ______________________________________                                        Monsanto Tunnel Tests of Wet Process Hardboard                                Containing Various Levels of Alumina Trihydrate                               and Ammonium Pentaborate                                                                   Monsanto tunnel Flamespread Length                               Composition*   without                                                        (% by weight)  borate add with borate add**                                   ______________________________________                                        Alumina trihydrate (25%)                                                                     231/2"     181/2"                                              Fiber (75%)                                                                   Alumina trihydrate (33%)                                                                     211/2"     16"                                                 Fiber (67%)                                                                   Alumina trihydrate (50%)                                                                     141/2"     11"                                                 Fiber (50%)                                                                   ______________________________________                                         *The fiber was the wet process interior type described in Table X.            **The ammonium pentaborate add level was 3-4%. A vacuum-pressure              impregnation was used.                                                   

The surprising nature, and the synergistic character of the combinationof alumina trihydrate and a source of B₂ O₃ is shown in Table V whereina board containing alumina trihydrate plus diammonium phosphate iscompared to a board containing alumina trihydrate and boric acid. As setforth in the Table V, diammonium phosphate is at least as effective asboric acid alone in reducing the flamespread index of a fiberboard.However, when added to a board containing alumina trihydrate, thecombination of alumina trihydrate with boric acid reduced theflamespread length from 141/2 inches to about 101/2 inches whereas thecombination of alumina trihydrate with diammonium phosphate increasedthe FSI from 141/2 inches to 161/2 inches. The results with othercompounds capable of providing B₂ O₃ at a temperature below about 400°F. were also synergistic and surprising.

                                      Table V                                     __________________________________________________________________________    A Comparison of the Effectiveness of Diammonium Phosphate and Boric Acid      in Dry Process Hardboard                                                                   Monsanto Tunnel Flamespread Length                               Composition*          With 5.5% Diammonium                                                                      With 5.5% Boric                             (% by weight)                                                                              No Chemical Add                                                                        Phosphate Add                                                                             Acid Add                                    __________________________________________________________________________    Fiber (100%) Traveled length                                                                        Traveled length of                                                                        Traveled length                                          of tunnel in 1                                                                         tunnel in 2 minutes                                                                       of tunnel in 2                                           minute 45 seconds                                                                      35 seconds  minutes 45 seconds                          Alumina trihydrate (50%)                                                                   141/2 "  161/2 "     101/2 "                                     Fiber (50%)                                                                   __________________________________________________________________________     *The fiber was the dry process pressure refined type described in Table X                                                                              

Further evidence of this synergistic effect is shown in Table VI whichlists ASTM E84 test results for boards containing various levels ofalumina trihydrate and borax:boric acid (1:1). The borax is notessential, but prevents undue hydrolysis on the surface of the board inlocalized areas, and aids in solubilizing the boric acid.

                  Table VI                                                        ______________________________________                                        ASTM E84 Tests of Various Levels of Alumina                                   Trihydrate and Borax:Boric Acid in Hardboard                                                   ASTM E84 test results                                        Composition*       Flamespread                                                (% by weight)      index        Smoke                                         ______________________________________                                        Fiber (100%)       174          250                                           Fiber (99%)        164          134                                           Borax:boric acid (1%)                                                         Fiber (98%)        151          142                                           Borax:boric acid (2%)                                                         Fiber (96%)        124          51                                            Borax:boric acid (4%)                                                         Fiber (95%)        115          55                                            Borax:boric acid (5%)                                                         Fiber (94%)        118          47                                            Borax-boric acid (6%)                                                         Fiber (78%)        103          --                                            Alumina trihydrate (22%)                                                      Fiber (70%)         96          62                                            Alumina trihydrate (30%)                                                      Fiber (50%)         38           0                                            Alumina trihydrate (50%)                                                      Fiber (71%)         60          13                                            Alumina trihydrate (23%)                                                      Borax:boric acid (6%)                                                         Fiber (66%)         44           0                                            Alumina trihydrate (28%)                                                      Borax:boric acid (6%)                                                         Fiber (60%)         21           0                                            Alumina trihydrate (28%)                                                      Borax:boric acid (12%)                                                        Fiber (48%)         18          --                                            Alumina trihydrate (47%)                                                      Boric acid (5%)                                                               ______________________________________                                         *The borax:boric acid was a 1:1 blend.                                   

As these results indicate, a substantial percentage of aluminatrihydrate can be replaced by a small percentage of about 1/2-7% byweight of a boron compound, such as boric acid, capable of providing B₂O₃ at a temperture below about 400° C., and particulatly at or below thecombustion temperature of the combustible component in the boardcontaining alumina trihydrate. For example a Class II product having aflamespread index of about 60 would, if feasible, require approximately40-45 parts by weight alumina trihydrate. A Class II board having thesame flame spread index can be manufactured while including only 23parts by weight alumina trihydrate with the addition of 3% B₂ O₃ formingcompound. In this manner about 40-50% by weight of alumina trihydratecan be economically saved while providing a product having substantiallyincreased strength. Similarly a product which would need 65 parts byweight alumina trihydrate to produce a Class I product (having a FSIless than about 25) when manufactured in accordance with the presentinvention needs only about 28 parts alumina trihydrate with the additionof 6 parts by weight B₂ O₃ forming compound. Thus, by the addition of 6parts by weight of a B₂ O₃ source, 37 parts by weight alumina trihydratecan be economically saved while providing a much stronger product.

The active boron component necessary in combination with the aluminatrihydrate to achieve synergistic flamespread reduction is B₂ O₃. The B₂O₃ can be provided by a number of boron containing compounds such asboric acid, and the ammonium borates such as the bi and pentaborates.Each of these B₂ O₃ -yielding compounds emits a green flame when theboard containing the compound is subjected to fire tests. To achieve thefull advantage of the present invention the boron compound should becapable of yielding B₂ O₃ at a temperature at or below the pyrolysistemperature of the combustible component in the board. Both boric acidand the ammonium borates are capable of providing B₂ O₃ below about 400°C. and at the approximate pyrolysis temperature of cellulosic fibers.Further, alumina trihydrate dehydrates at about the same temperture.More stable B₂ O₃ sources, such as the zinc borates and sodium borates,require much higher temperatures than 400° C. to yield B₂ O₃ and do notexhibit the synergism noted with boric acid and ammonium borates. Thesource of B₂ O₃ should be present in the final product in an amount ofat least about 1/2% by weight of the product so that the flame-spreadwill be sufficiently reduced. Additions greater than about 10% by weightmay cause excessive water sensitivity of the final product. This isparticularly true when borax is used as a buffer since it will alsocause increased water sensitivity of the final product. Charring of theman-made fibrous product may also become excessive at high boratelevels. It is therefore not recommended that the source of B₂ O₃ bepresent in an amount which will yield greater than about 10% by weightB₂ O₃ in the final product.

Alumina trihydrate, also known as Gibbsite α-alumina trihydrate,hydrated alumina, and aluminum trihydroxide, should be present in theman-made fibrous product in an amount of at least about 10% by weight toobtain the full advantage of the invention. Alumina trihydrate is idealfor incorporation in man-made fibrous boards since it is stable atordinary consolidation (hot-pressing) conditions and releases water atapproximately the pyrolysis temperature of man-made fibrous products.The chemically bonded water molecules of the alumina trihydrate willseparate, causing the alumina to dehydrate at elevated temperatures inthe range of about 200°-500° C. The dehydration is an endothermicreaction requiring a substantial amount of heat and materially aids inflamespread reduction. Further, the separated water molecules andresulting steam production aids in flamespread reduction by mixing ofsteam with the flammable gases to provide dilution thereof. A wide rangeof alumina trihydrate particle sizes and purity grades can be obtainedfrom various manufacturers, and some typical properties of these aluminatrihydrates are given in Table VII.

                                      Table VII                                   __________________________________________________________________________    Typical Properties of Alcoa Alumina Trihydrates                               Grades          C-30                                                                              C-31                                                                              C-31                                                                              C-37                                                                              C-330                                                                             C-331                                                                             705                                   Typical properties*     coarse                                                __________________________________________________________________________    Al.sub.2 O.sub.3                                                                            % 64.9                                                                              65.0                                                                              64.9                                                                              64.2                                                                              65.0                                                                              65.0                                                                              64.1                                  SiO.sub.2     % 0.012                                                                             0.01                                                                              0.01                                                                              0.07                                                                              0.02                                                                              0.01                                                                              0.04                                  Fe.sub.2 O.sub.3                                                                            % 0.008                                                                             0.003                                                                             0.004                                                                             0.002                                                                             0.025                                                                             0.006                                                                             0.01                                  Na.sub.2 O    % 0.40                                                                              0.15                                                                              0.20                                                                              0.42                                                                              0.30                                                                              0.15                                                                              0.60                                  Moisture (110 C)                                                                            % 0.10                                                                              0.04                                                                              0.04                                                                              --  0.40                                                                              0.40                                                                              0.60                                  Bulk density, loose, lb/ft.sup.3                                                              75-85                                                                             60-70                                                                             70-80                                                                             50-60                                                                             44.00                                                                             44.00                                                                             5-9                                   Bulk density, packed, lb/ft.sup.3                                                             95-105                                                                            75-85                                                                             90-100                                                                            60-70                                                                             77.00                                                                             77.00                                                                             6-12                                  Specific Gravity                                                                              2.42                                                                              2.42                                                                              2.42                                                                              2.53                                                                              2.42                                                                              2.42                                                                              2.40                                  Seive analysis (cumulative)                                                    On 100 mesh  % 5-20                                                                              0-1 0-10                                                                              6-12                                                                              --  --  --                                     On 200 mesh  % 65-90                                                                             5-10                                                                              40-80                                                                             30-60                                                                             --  --  --                                     On 325 mesh  % 90-98                                                                             30-65                                                                             85-97                                                                             75-95                                                                             1.0 1.0 0.04                                   Through 325 mesh                                                                           % 2-10                                                                              35-70                                                                             3-15                                                                              5-25                                                                              99.0                                                                              99.0                                                                              99.96                                 __________________________________________________________________________     *Data was taken from Alcoa Product Data Sheet-Chemicals, section GA4A,        November 1, 1972                                                         

The coarser, less pure grades have been found adequate for use in boththe wet and dry process. Without the addition of the B₂ O₃ yieldingcompound, however, it is very difficult to manufacture a man-madealumina trihydrate containing fibrous product having adequate physicalproperties for commercial production since such a product must containapproximately 45% alumina trihydrate for a Class II product and 60-70%for Class I product. In accordance with the present invention, bycombining a source of B₂ O₃ with the alumina trihydrate, a Class IIflamespread rated man-made fibrous product can be manufactured havingless than about 45% alumina trihydrate for a Class II product and moreparticularly less than 30% alumina trihydrate. A Class I product wouldrequire 65% alumina trihydrate without the B₂ O₃ source but only needs30-35 parts alumina trihydrate with the B₂ O₃ source.

The point of addition of the alumina trihydrate and the B₂ O₃ sourcedepends upon the method used in manufacturing the board. When theman-made fibrous product of the present invention is manufactured by thewet process, the alumina trihydrate particles are slurried with thefibrous material, and the wetlap formed and dewatered on a waterpervious support member, such as a fourdrinier wire, to a moisturecontent in the range of about 25-75% by weight. A source of B₂ O₃, suchas boric acid, is then deposited onto the surface of the partiallydewatered mat. It is preferred to add the source of B₂ O₃ to the wetlapsurface when it has a moisture content in the range of about 40-60% byweight. The source of B₂ O₃ is deposited onto the surface of thepartially dewatered mat in any manner known in the art, for example byallowing the source of B₂ O₃ to flow over a wire positioned above thesurface of the partially dewatered wetlap as it is continuously formed.The source of B₂ O₃ penetrates into the partially dewatered mat innormal drying and hot pressing operations used to form and consolidatethe man-man fibrous product. The fibrous product is dried in any mannerknown in the art, such as in drying ovens or during hot pressing at, forexample, 750 psi and 325° F., to provide a product having a moisturecontent less than about 9% by weight and preferrably in the range ofabout 5-7% by weight. If desired, the product can be completely dried toprovide a bone dry product, however, the bone dry product will pick upambient moisture so that when the moisture content reaches equilibriumit is commonly in the range of about 5-7% by weight.

Another method of providing a source of B₂ O₃ in the man-made fibrousproduct made by the wet process is by impregnating the fibrous productafter consolidation. Impregnation after consolidation is preferred sinceby depositing the B₂ O₃ source onto the surface of the wet lap, aportion of the B₂ O₃ source is lost, either by drainage through the wetmat or by vaporization during hot pressing. When the B₂ O₃ source isdeposited onto the surface of the wet lap, it must be provided in anamount greater than the desired final amount of B₂ O₃. Generally, anamount of about 20-30% of the B₂ O₃ source is lost as a result ofdrainage or vaporization during consolidation so that up to about 10%weight of the B₂ O₃ source can be added to the surface of the wet lap.Little difference in flamespread reduction is noted with respect to themethod of B₂ O₃ source addition. Flamespreads of boards made withadditions of a B₂ O₃ source to the wetlap as compared to boards madewith impregnations of the B₂ O₃ source are set forth in Table VIII.

                  Table VIII                                                      ______________________________________                                        A Comparison of Post-Press Adds of Borates to                                 Wet Process Boards Containing Alumina Trihydrate                              Composition*                                                                              Method of Borate                                                                            Monsanto Tunnel                                     (% by weight)                                                                             Addition      Flamespread Length                                  ______________________________________                                        Alumina trihydrate (30%)                                                                  no borate added                                                                             traveled length of                                  Fiber (60%)               tunnel in 3 minutes                                                           45 seconds                                          Alumina trihydrate (30%)                                                                  pre-press add to                                                                            16"                                                 Borax: boric acid                                                                         wetlap                                                              (6% add retained)                                                           Fiber (70%)                                                                   Alumina trihydrate (32%)                                                                  post-press    161/2 "                                             Borax:boric acid                                                                          impregnation                                                        (5.5% add retained)                                                         Fiber (68%)                                                                   ______________________________________                                         *The fiber was the wet process interior type described in Table X. The        borax:boric acid was a 1:1 blend by weight. Addition of borates given in      by weight of alumina trihydrate plus fiber.                              

When the man-made fibrous product of the present invention ismanufactured by the dry process, the alumina trihydrate is added at anyconvenient point before the dry mat is formed and before it isconsolidated under heat and pressure, for example at 600 psi and 425° F.It is generally preferred to add the alumina trihydrate at a point whichallows intimate mixing of the alumina trihydrate and wood fiber in thesystem air currents prior to felting on the forming wire. For example,the alumina trihydrate can be added to a conduit used to convey thefibers to the forming wire. Resin and wax can be added to the fiber orthe fiber/alumina trihydrate blend by methods well known to those in theart. The B₂ O₃ source is added either during primary manufacture or byimpregnation after consolidation of the product. When added to the woodfiber during primary manufacture, the B₂ O₃ source can be premixed inpowdered form with the alumina trihydrate or can be added separately atany convenient point in either powder or solution form. When the B₂ O₃source is added by impregnation after the product has been pressed,vacuumpressure impregnation techniques, well known to those in thetrade, are used. The method or order of the addition of the B₂ O₃ sourceand the alumina trihydrate is not critical. More of the synergisticmixture of alumina trihydrate and the B₂ O₃ source may be added to thecenter of the product than the faces, but the exact distribution is notcritical with respect to the uniqueness of the synergism noted.

Alumina trihydrate losses during formation play an important part in thecosts, and laboratory work has shown that retention can be improvedsignificantly with larger particle sizes and finer fiber types. Almost100% recovery of the alumina trihydrate from the pit water is possiblesince it quickly settles out. Alumina trihydrate formation losses withvarious fiber and alumina trihydrate sizes are set forth in table IX.

                  Table IX                                                        ______________________________________                                        Wet Process Formation Losses With Various Fiber                               And Alumina Trihydrate Sizes*                                                                    With alumina trihydrate                                                   No alumina                                                                              ultra                                                Fiber Type     trihydrate                                                     fine           medium    coarse                                               ______________________________________                                        Coarsest                                                                      Exterior grade wet process                                                                   3%                                                             fiber (Masonite process)                                                                               --                                                                                   30%                                                                                  --                                     Interior grade wet process                                                                   3%                                                             fiber (Masonite process)                                                                               30%                                                                                  25%                                                                                  14%                                    Interior grade dry process                                                                   9%                                                             fiber (pressure refined)                                                                               --                                                                                   21%                                                                                  --                                     ______________________________________                                         Finest                                                                        *The losses were determined on blends of equal amounts of fiber and           alumina trihydrate, plus a 3% add of phenol formaldehyde resin and a 1        1/2% add of wax. The blends without alumina trihydrate contained only         fiber plus a 3% add of resin and a 1 1/2% add of wax.                    

The fiber distributions of typical fiber blends used in the manufactureof the product of the present invention are set forth in Table X.

                                      Table X                                     __________________________________________________________________________    Fiber Size Distributions                                                                   Percentage of Material on U.S. Seive Number:*                    Fiber Type   6  9  20  40  100 170 pan                                        __________________________________________________________________________    Exterior grade wet process                                                                 0.8%                                                                             5.9%                                                                             29.3%                                                                             61.7%                                                                             84.6%                                                                             91.7%                                                                             100.1%                                     fiber (Masonite process)                                                      Interior grade wet process                                                                 -- 4.8%                                                                             29.1%                                                                             57.6%                                                                             81.8%                                                                             90.6%                                                                             100.0%                                     fiber (Masonite process)                                                      Interior grade dry process                                                                 -- --  6.6%                                                                             25.1%                                                                             73.7%                                                                             84.5%                                                                              99.9%                                     fiber (pressure refined)                                                      __________________________________________________________________________     *Percentages determined after 30 minute runs of 40 gram samples in a          commercial Ro-Tap.                                                       

We claim:
 1. A fibrous product comprising combustible fibers, an aluminahydrate in an amount in the range of 10-60% by weight of said product,and a boron containing compound selected from the group consisting ofboric acid, mixture of boric acid and borax, and an ammonium borate,said borate containing compound being present in said product in anamount of at least 1/2% by weight of said product.
 2. A product asdefined in claim 1 wherein the boron containing compound comprises boricacid.
 3. A product as defined in claim 1 wherein the boron containingcompound comprises an ammonium borate.
 4. A product as defined in claim3 wherein the ammonium borate comprises ammonium pentaborate.
 5. Aproduct as defined in claim 3 wherein the ammonium borate comprisesammonium biborate.
 6. A product as defined in claim 1 wherein the boroncontaining compound comprises 1-10% by weight of said product.
 7. Aproduct as defined in claim 1 wherein the combustible fibers areincluded in said product in an amount in the range of 20-90% by weightof the product.
 8. A product as defined in claim 1 wherein the borax isincluded in an amount of about 1-10% by weight of said product.
 9. Aproduct comprising cellulosic fibers, 20-60% by weight aluminatrihydrate, and a boron compound selected from the group consisting ofboric acid, mixture of boric acid and borax, and an ammonium borate,said boron compound contained in said product in an amount of at least1/2% by weight of said product.
 10. A product as defined in claim 9wherein the boron compound comprises ammonium pentaborate.
 11. A productas defined in claim 9 wherein the boron compound comprises ammoniumbiborate.
 12. A method of making a fibrous product having a reducedflamespread comprising:mixing together a slurry of components comprisingwater and combustible fibers, depositing at least a portion of saidslurry onto a water pervious support member to form a wet mat, adding tothe wet mat alumina trihydrate, and a source of B₂ O₃ effective inreducing the flamespread of the fibrous product, said source of B₂ O₃selected from the group consisting of boric acid, mixture of boric acidand borax, and an ammonium borate, wherein said alumina trihydrate isadded in an amount sufficient to provide 10-60% alumina trihydrate insaid fibrous product and wherein said source of B₂ O₃ is added in anamount sufficient to provide at least 1/2% B₂ O₃ in said fibrousproduct, and consolidating the fibrous product.
 13. A method as definedin claim 12 wherein said source of B₂ O₃ is added to said slurry ofcomponents.
 14. A method as defined in claim 12 wherein said source ofB₂ O₃ is added to said wet mat when said wet mat contains about 20-60%by weight water.
 15. A method as defined in claim 14 wherein saidalumina trihydrate is added to said slurry of components.
 16. A methodas defined in claim 14 wherein the source of B₂ O₃ is deposited onto thewet mat in an amount sufficient to provide 1-5% by weight of said B₂ O₃source in the consolidated product.
 17. A method as defined in claim 12wherein the source of B₂ O₃ is deposited on the surface of the wet matin an amount of about 5-8% by weight of solids in the wet mat.
 18. Amethod of forming a fibrous product having a reduced flamespread indexcomprising:mixing together cellulosic fibers, alumina trihydrate, and abinder, adding a source of B₂ O₃ effective in reducing the flamespreadof the fibrous product, said source of B₂ O₃ selected from the groupconsisting of boric acid, mixture of boric acid and borax, and anammonium borate, wherein said alumina trihydrate is added in an amountsufficient to provide 10-60% alumina trihydrate in said fibrous productand wherein said source of B₂ O₃ is added in an amount sufficient toprovide at least 1/2% B₂ O₃ source in said fibrous product, air-layingthe mixture onto a support member, and consolidating the mixture withheat and pressure to form a fibrous product having a reduced flamespreadindex.
 19. A method as defined in claim 18 wherein the source of B₂ O₃is added to the fibers prior to air-laying the mixture onto the supportmember.
 20. A method as defined in claim 18 wherein the source of B₂ O₃is impregnated into the fibrous product after consolidation of saidproduct.
 21. A method of making a fibrous product having a reducedflamespread comprising:mixing together a slurry of components comprisingwater, alumina trihydrate and combustible fibers, depositing at least aportion of said slurry onto a water pervious support member to form awet mat, removing at least a portion of the water from said wet mat, andadding thereto a source of B₂ O₃ effective in reducing the flamespreadof the fibrous product, said source of B₂ O₃ selected from the groupconsisting of boric acid, mixture of boric acid and borax, and anammonium borate, wherein said alumina trihydrate is combined in saidslurry in an amount sufficient to provide 10-60% alumina trihydrate insaid fibrous product and wherein said source of B₂ O₃ is added in anamount sufficient to provide at least 1/2% B₂ O₃ source in said fibrousproduct.
 22. A method of making a fibrous product having a reducedflamespread comprising:mixing together a slurry of components comprisingwater and combustible fibers, depositing at least a portion of saidslurry onto a water pervious support member to form a wet mat, removinga portion of said water from said wet mat to form a partially dried wetmat, adding alumina trihydrate to said partially dried wet mat when saidpartially dried wet mat contains water in an amount of 20-60% by weight,wherein said alumina trihydrate is added in an amount sufficient toprovide 10-60% alumina trihydrate in said fibrous product, and adding asource of B₂ O₃ effective in reducing the flamespread of the fibrousproduct, said source of B₂ O₃ selected from the group consisting ofboric acid, mixture of boric acid and borax, and an ammonium borate,wherein said source of B₂ O₃ is added in an amount sufficient to provideat least 1/2% B₂ O₃ source in said fibrous product.
 23. A method asdefined in claim 21 wherein said source of B₂ O₃ is impregnated intosaid fibrous product after consolidation thereof.
 24. A method asdefined in claim 22 wherein said source of B₂ O₃ is impregnated intosaid fibrous product after consolidation thereof.
 25. A method offorming a fibrous product having a reduced flamespread indexcomprising:mixing together cellulosic fibers, alumina trihydrate, and abinder, said alumina trihydrate added in an amount such that afterconsolidation of the fibrous product, said product contains aluminatrihydrate in an amount of 10-60% by weight of said product, air-layingthe mixture onto a support member, consolidating the mixture under heatand pressure to form a fibrous product, and adding a source of B₂ O₃ tosaid consolidated fibrous product, effective in reducing the flamespreadof the fibrous product, said source of B₂ O₃ selected from the groupconsisting of boric acid, mixture of boric acid and borax, and anammonium borate, wherein said source of B₂ O₃ is added in an amountsufficient to provide at least 1/2% B₂ O₃ source in said fibrousproduct.